1. Field of the Invention
The present invention relates to a data communication system transmitting and receiving a data stream including a plurality of frames and a plurality of flags thereamong.
2. Description of Related Art
To detect the boundaries of the frames, flags such as HDLC (High-Level Data Link Control) flags and PN (Pseudo Noise) flags, have been used. As taught in the U.S. Pat. No. 6,266,349 xe2x80x9cMETHOD AND APPARATUS FOR DETECTING FRAME IN DATA STREAMxe2x80x9d, Fukui et al on Jul. 24, 2001 and assigned to Oki Electric Industry Co., Ltd, a communication system using such flags might cause a flag emulation. A flag emulation is defined as follows: a bit stream similar to the pattern of the flags in a frame which forces the receipt apparatus to misunderstand the bit stream as a flag; and consequently the frame fails to be completely regenerated by the receipt apparatus. Hereinafter, such a bit stream may be referred to as a xe2x80x9cfalse flagxe2x80x9d or xe2x80x9cfalse patternxe2x80x9d.
In general, as shown in FIG. 8, each frame is constructed of a header and a data, wherein the header is indicated by xe2x80x9cHxe2x80x9d, and flags are indicated by xe2x80x9cFxe2x80x9d. There are cases when error check data such as CRC(Cyclic Redundancy Check) data may be added. If the data has a false flag (Fxe2x80x2), the false flag (Fxe2x80x2) leads to a flag emulation. Namely, the receipt apparatus misunderstand the false flag as the boundary of the frame which is now receiving. So, the receipt apparatus processes the remaining data as data xe2x88x922 without a header or some other non-effective data. Therefore, the receipt apparatus has failed to acquire the contents of the remaining data as well as the contents of the false flag (Fxe2x80x2).
As countermeasures against this problem, the above patent application has disclosed several techniques. Among them, there is an insertion of a xe2x80x9cmarkxe2x80x9d after each false flag to represent the presence of the false flag, which enables avoidance of a flag emulation. This technique, however, makes rather complicated the configuration of both the transmission apparatus and the receipt apparatus, in comparison with the prior art; besides, it does not meet ITU-T Recommendation H. 233/Annex Axe2x80x94Multiplexing Protocol for Low Bitrate Multimedia Communication over Low Error-Prone Channels (1998).
The object of the present invention is to provide a data communication apparatus that is robust to flag emulation.
According to one aspect of the present invention, there is provided a data communication apparatus which prepares a frame signal including a plurality of frames and a plurality of flags enabling recognition of the plurality of frames, wherein each frame includes a bit stream and an additional data and each flag is laid between two frames adjacent to each other, the data communication apparatus comprising: a detecting circuit which detects a false pattern similar to the pattern of the flags in each bit stream; and a preparing circuit which prepares at least one frame using the portion of the bit stream other than the detected false pattern.
According to another aspect of the present invention, there is provided data communication apparatus, wherein the preparing circuit includes a additional data preparing circuit which prepares an additional data for the portion, and a frame preparing circuit which prepares a frame using the additional data and the portion.
According to further another aspect of the present invention, there is provided a data communication apparatus, wherein the detecting circuit detects a false pattern in the prepared frame, and the preparing circuit prepares another frame using the portion of the prepared frame other than the detected false pattern therein.
According to still another aspect of the present invention, there is provided a data communication apparatus which prepares a frame signal including a plurality of frames and a plurality of flags enabling recognition of the plurality of frames, wherein each frame includes a bit stream and an additional data and each flag is laid between two frames adjacent to each other, the data communication apparatus comprising: a detecting circuit which detects a false pattern similar to the pattern of the flags in each bit stream; a dividing circuit which divides the bit stream into a first and a second portions so that the first portion includes a part of the detected false pattern and the second portion includes the other part thereof; and a preparing circuit which prepares a frame using the first portion and prepares another frame using the second portion.
According to still another aspect of the present invention, there is provided a data communication apparatus which receives a frame signal including a plurality of frames, a plurality of first flags, and a second flag, wherein each frame includes a bit stream and an additional data, each first flag is laid between two frames adjacent to each other, and the second flag is laid at the predetermined distance from the false pattern similar to the pattern of the flags in a bit stream, the pattern of the firsts flags and the pattern of the second flag being identical with each other, the data communication apparatus comprising: a detecting circuit which detects a plurality of patterns similar to the pattern of the flags in the frame signal; a recognizing circuit which recognizes a first pattern and a second pattern at the predetermined distance from the first pattern among the plurality of detected patterns, as the false pattern and the second flag; and a deleting circuit which deletes the plurality of the first flags and the second flags from the frame signal to prepare the plurality of frames.